1. Gelation of Konjak Mannan
Konjak is a traditional foodstuff and it has been prepared, roughly speaking, by swelling konjak potato or konjak powder with water, kneading, adding an alkali agent such as lime, kneading and then heating. By gelation of konjak mannan, konjak has a peculiar viscoelasticity.
On the other hand, konjak involves known problems in that (1) an alkaline odor inherent to konjak due to the alkali treatment is generated, (2) water is easily released, (3) it is not resistant to freezing, (4) skill is required for its preparation; in particular, skill is needed for addition of the alkali agent and kneading and, because the reaction rapidly progresses, molding and finishability become poor, etc. For purposes of alleviating the alkali odor, preventing release of water, imparting freezing resistance, improving preparation operations, etc., many investigations have been made. These improvements can be roughly classified as alleviation of the alkali odor by reducing the amount of an alkali agent added, neutralizing after the addition of an alkali agent, treating to remove the alkali, etc. (Published Unexamined Japanese Patent Application Nos. 1752/74, 26882/80, 144070/81, 166965/82, 36366/83, 51866/83, etc.); prevention of water release by adding carrageenan or other agents to prevent release of water (Published Unexamined Japanese Patent Application Nos. 22650/73, 75759/74, etc.); imparting freezing resistance using cyclodextrin and carrageenan in combinaton (Published Unexamined Japanese Patent Application No. 216662/83); etc. In addition, many investigations on improvements have been reported.
These investigations are all, without exception, based on the presumption that gelation of konjak is effected only by treatment with an alkali. Namely, as far as konjak is concerned, it is considered that alkali treatment is mandatory. In fact, no report other than the following example is known with respect to gel formation of konjak mannan without treating with alkali.
Published Unexamined Japanese Patent Application No. 1754/74 discloses that konjak having a good gel strength is obtained without performing an alkali treatment. However, as shown in a Reference Example hereinafter described, the gel strength of the gel of carrageenan and purified konjak powder (weight ratio of 3:2) is reduced to approximately 2/3 with K-carrageenan and approximately 1/10 with I-carrageenan, as compared to that of carrageenan alone (an aqueous solution thereof which is gelled by itself).
Further, the finding that reversible konjak is obtained at a pH lower than a neutral pH is described in Published Unexamined Japanese Patent Application No. 116651/83. Even in this method, the upper limit of the pH is 10 while the lower limit of the pH is set at +0.2; even where the concentration of konjak powder is 4%, a weak, paste-like gel is merely obtained unless an alkali for solidifying konjak, e.g., sodium hydroxide, calcium carbonate, etc., is used in combination.
2. Konjak Mannan-Containing Reversible Gel
Konjak mannan gel obtained by adding water to konjak mannan and treating the thus swollen sol with an alkali is an irreversible gel which shows a strong and elastic solidified state both at room temperature and at high temperatures, about 100.degree. C.
To the contrary, the reversible konjak described in Published Unexamined Japanese Patent Application No. 203467/84 in addition to Published Unexamined Japanese Patent Application No. 116651/83, supra, is known as a reversible konjak mannan gel which varies from a liquid to solidified state depending upon the change in temperature.
(a) Published Unexamined Japanese Patent Application No. 116651/83.
A thermally reversible konjak is disclosed which exists as a liquid to a paste-like state at normal temperature and shows a konjak-like solidified state at 60.degree. C. or higher. This gel has opposite properties to normal gels.
______________________________________ Process for preparation: konjak potato or konjak powder - (swelling by addition of water) - (adding a catalyst*) - (kneading**) - heat treatment at 70.degree. C. or higher) ______________________________________ *Catalyst: (1) Sodium citrate, sodium tartarate, sodium malate, sodium lactate, sodium acetate, sodium succinate, etc., or a mixture thereof is used alone or in combination with calcium carbonate or magnesium carbonate. (2) Sodium polyphosphate, potassium polyphosphate, sodium pyrophos- phate, potassium pyrophosphate, sodium metaphosphate, potassium metaphosphate, sodium (mono-, di-, tri-)phosphate, potassium (mono-, di-, tri-)phosphate, ammonium (mono-, di-)phosphate, etc., or a mixture thereof is used alone or in combination with calcium carbonate or magnesium carbonate. (3) Sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, calcium hydroxide, calcium carbonate, sodium sulfite, magnesium carbon- ate, sodium hydrogen carbonate, ammonium hydrogen carbonate, ammonium carbonate or a mixture thereof. ______________________________________ **pH: pH prior to the addition of the catalyst: +0.2 to 10
(b) Published Unexamined Japanese Patent Application No. 203467/84.
A thermally reversible konjak is disclosed which exists in a semi-solid to a solid state at normal temperature and, in a liquid.about.paste.about.semi-solid state prior to chilling to 10.degree. C.
Process for preparation: konjak potato or konjak powder--(swelling by addition of water)--(adding a catalyst*)--(kneading**)--(heat treatment at 50.degree. C. or higher)--(neutralization or treatment for removing alkali) FNT *same catalyst as described in (1) above **pH of 10.01 to 12.20
3. Gelation of Xanthane Gum
It is known that an aqueous solution of xanthane gum does not form a gel by itself. However, when used in combination with locust bean gum, which is a galactomannan type gum, or a polysaccharide composed of galactose and mannose, a stronger gel is formed. However, it is also known that, when it is used in combination with guar gum which is a galactomannan type gum likewise, a remarkable increase in viscosity is exhibited but no gelation occurs.
On the other hand, no report on gelation using konjak mannan or glucomannan composed of glucose and mannose in combination with xanthane gum has been found. Namely, as described above, the alkali treatment is considered to be essential for gelation of konjak mannan. As a matter of fact, the alkali treatment is performed in the system involving use of xanthane gum in combination with konjak mannan (Published Examined Japanese Patent Application No. 45173/80, Published Unexamined Japanese Patent Application No. 216662/83).